Just a small point I'd like add to this debate having spent many years working in the petrochemical industry. Yes there are slight differences between brands, but the refining process has to conform to set standards within a given tolerance. The chemical makeup of the fuel is lab tested on a regular basis during production and it's all basically the same in the U.K.
Some additives such as detergents and silicates are added to fuels very late on in the process - in fact this happens just before it goes to the on-site storage tanks.
Most of the supermarket delivery trucks collect road fuel from the nearest refinery or oil depot. Bunsfield oil depot (where the big fire was) received various fuels from various refineries by long distance pipeline, so you can't really be sure where it came from.
As the additives have a different density than the fuel itself, the best fuel is usually obtained from busy forecourts as they have a regular turnround of fresh fuel and it doesn't get a chance to separate out in the storage tanks.

A mate that works for Cepsa in Algeciras, Spain (Close to Gibraltar) told me exactly the same thing. He says the additives may vary the efficiency, but in place of busy petrol stations._________________Land Rover Ligero 1982 2,5 n/a Diesel

Up to 2 weeks ago I was running a Skoda 1.6 diesel estate. I found if I ran her on a Super diesel she ran quieter with better performance and about 12-15% more mpg. OH found similar results with his Movano 2.5 van. Price is usually 8% more than normal diesel. The Landy came with 1/2 tank of diesel, so it will take a few fills to clear that out of the system. Will be interesting to see what mpg I can then get on the Wolf. I usually stick with BP or Shell._________________Growing Old Disgracefully